A conventionally known gas sensor is attached to an exhaust path of an internal combustion engine such as an automotive engine for detecting the concentration of NOx (nitrogen oxide) in exhaust gas (gas to be measured) (refer to Patent Documents 1 and 2). Patent Documents 1 and 2 describe a gas sensor element which partially constitutes the gas sensor and includes a plate-like solid electrolyte body having oxygen ion conductivity, a first electrode provided on the front or back surface of the solid electrolyte body, a first lead connected to the first electrode, a second electrode provided on the front or back surface of the solid electrolyte body, a measuring chamber (second chamber) which is disposed in opposition to the first electrode and into which gas to be measured is introduced, and a reference oxygen chamber disposed in opposition to the second electrode. The gas sensor element is configured such that, as a result of movement of oxygen ions stemming from NOx contained in the gas-to-be-measured introduced into the measuring chamber, from the measuring chamber to the reference oxygen chamber through the solid electrolyte body, a current corresponding to the concentration of oxygen stemming from the NOx flows between the first electrode and the second electrode.
According to Patent Documents 1 and 2, in order to ensure oxygen pumping performance, the first electrode is formed to be porous. As a result, the first electrode becomes oxygen-permeable. Meanwhile, in order to improve electrical conductivity, high density per unit area is desired; thus, the first lead is formed to be dense. As a result, the first lead becomes oxygen-impermeable.